A safety switch arrangement of this type is known from German Patent 198 15 150 A1. This patent discloses an arrangement of sensors for monitoring a tool, wherein the tool is put into operation in dependence on the switching states of the sensors. The sensors function as slaves in a bus system operating according to the master-slave principle. The master is a control unit, which cyclically polls the slaves under predetermined addresses.
The sensors are light barriers having, respectively, one transmitter and one receiver, wherein each transmitter transmits light rays with an individual coding to the associated receiver. A redundant evaluation unit is connected to the bus system, which continuously monitors the signals transmitted via the bus system. The tool is made operational by the evaluation unit only in case of an error-free identification of the coding transmitted by the receivers via the bus system.
The arrangement has the advantage that errors in the transmission of signals via the bus system can be detected with a high degree of certainty by evaluating the coding in the evaluation unit.
For this, the sensors advantageously are not required to have a redundant design, but can still monitor the transmission of signals via the bus system with the level of safety that is required for using the arrangement in the area of protection of persons.
In place of sensors, actuators such as emergency-off switches can, in principle, also be connected as slaves in the bus system. Emergency-off switches of this type are designed to have two channels for safety reasons. Thus, each emergency-off switch has two contacts which are opened when the emergency-off switch is actuated. The state of a non-actuated emergency-off switch corresponds to the interference-free operation. Given a clear beam path and this operative state, the codings are transmitted to the evaluation unit in the same way as for a light barrier. The codings can be transmitted, for example, in the form of a 4-bit wide numerical word for which the value changes continuously according to a specific sequence. If the emergency-off switch is activated, it corresponds to an interference that is identical to an object interfering with the beam path of a light barrier. In that case, the emergency-off switch transmits a fixed value for the numerical word, which differs from the codings, to the evaluation unit so that the tool is shut down. The value for the numerical word for a 4-bit numerical word, for example, is "0000".
It is sufficient to open up one contact of the emergency-off switch to shut down the tool. However, a possible defect in the second contact of the emergency-off switch is not detected in that case. If the second contact of the emergency-off switch also fails, this can result in danger to the operating personnel for the tool, since the tool can no longer be shut down with the emergency-off switch.